The production of a red wine initially involves removing the stalks from the grapes, which entails picking off the grape berries in order to eliminate the essentially ligneous stalks which could impair the fineness and fruity flavor of the wine. Pressing is subsequently carried out, for the purpose of bursting the berries and extracting the juice therefrom. Pressing must not be too intensive, to avoid crushing the skin or the cuticle and the pips, since this could also be detrimental to the quality of the wine.
At this stage of preparation, the grapes are then placed in a wine-making vat where the conversion of the juice into wine will take place. This conversion essentially involves the phenomenon of fermentation, during which the sugars are converted into ethyl alcohol and carbon dioxide. The juice or must becomes cloudy and heats up, and bubbles of carbon dioxide gas are released.
During fermentation, a liquid phase and a solid phase can be distinguished in the wine-making vat, the liquid phase consisting of the juice or must and the solid phase consisting of the skins or cuticles of the berries, or marc. Because of the release of bubbles of carbon dioxide gas, the marc forms a cake, or marc head, which floats above the juice.
In conjunction with the fermentation, steeping or exchanges of aromatic and coloring substances take place between the marc and the must, and this in fact will give the red wine the characteristics of taste and color peculiar to it. It is therefore important to assist these exchanges and also to prevent the possibility that the marc head will itself be damaged by oxidation. This explains the presence in conventional vats of grids or gratings keeping the marc head submerged, or the dipping operation which involves periodically breaking the marc and submerging it in the juice. The continuous moisturizing of the marc head by the juice assists its subsequent extraction from the vat.
When fermentation is completed, after a period of the order of five to eight days on average, the liquid phase or "first run" is drawn. The marc is extracted from the vat and subjected to a pressing operation to extract from it still the wine which it contains. This "second pressing wine", richer in tannin and dry extracts than the first run, is either treated separately or mixed together with the first run.
The wine-making method for obtaining white wine or white-wine making differs essentially from that for red wine in that, after the possible removal of the stalks, the grapes are subjected to the pressing operation, and only the juice is put in the fermentation vat in order to prevent any coloration by the skins of the berries, whether these come from red grapes with white juice or white grapes with white juice (wine called "blanc de blanc").
Whatever the wine-making method, the conversion of the juice into wine is based fundamentally on an anaerobic alcoholic fermentation or conversion of the natural sugars of the grape (glucose and fructose) into ethyl alcohol according to the reaction: EQU C.sub.6 H.sub.12 O.sub.6 .fwdarw.2(CO.sub.2)+2(C.sub.2 H.sub.5 OH).
This equation makes it possible to show the large amount of carbon dioxide gas which accompanies the formation of alcohol. In fact, a stoichiometric calculation weighted by a conversion efficiency of 90% makes it possible to see that the conversion of a juice into 1 litre of wine with a titer of 12.degree. of alcohol is accompanied by a release of approximately 50 normal liters of carbon dioxide gas.
However, this equation does not represent all the phenomena which occur during fermentation and which make it necessary for the vine grower to carry out a continuous check of the wine-making process.
Thus, for the fermentation to take place, the presence of certain yeasts is essential. These yeasts of the Saccharomyces type are naturally present on the berries, but their catalytic activity depends largely on the temperature of the medium. So that fermentation can begin, the temperature of the juice must not be below 15.degree. C., the ideal temperature being between 18.degree. C. and 20.degree. C. Furthermore, the fermentation reaction is exothermic, and therefore the temperature of the juice will increase during the process, thus risking being detrimental to the yeasts themselves. In fact, their growth is slowed sharply above 38.degree. C. and they are destroyed between 40.degree. C. and 50.degree. C. In this case, the conversion of the sugar into alcohol will stop and the content of residual sugar in the wine will be too high, at the expense of its percentage of alcohol, and this can also be conducive to the generation of acetic acid.
In fact, for making red wine, the temperature during fermentation is set between 25.degree. C. and 30.degree. C., which is an ideal range for the fermentation itself and also for the exchanges of substances by steeping.
For making white wine, the temperature is a little lower, to give this type of wine its particular characteristics, the temperature of the juice being maintained between 18.degree. C. and 25.degree. C.
These constraints are well known to those skilled in the art, and for that reason most of the modern wine-making vats comprise heat exchangers making it possible to heat or cool the juice in order to adjust its temperature to the requisite value.
Although the presence of some biochemical species, (such as the Saccharomyces yeasts, which moreover are naturally indigenous in the vineyards) is essential for the proper execution of the wine-making process, it is important to protect the fermentation medium against the uncontrolled infiltration of atmospheric air; in fact, oxidation and extraneous seeding with bacteria can give rise to undesirable parallel fermentations detrimental to the alcoholic fermentation or to plainly harmful reactions, such as, for example, acetic aerobic fermentation or the generation of acetic acid by acetic bacteria.
To meet these various requirements, wine-making installation must make it possible to adjust the temperature of the juice for the start of fermentation and during fermentation; mashing of the juice makes it possible to obtain a uniform temperature and also a mass distribution of the fermentation reactions. This is beneficial to the steeping of the marc and must, more especially for making red wine. Furthermore, the installation must allow wine-making in an anaerobic and aseptic medium and provide a flexibility of use, so that it can be adapted to the traditional process particular to the vineyard, and be simple and inexpensive to operate and maintain.
FR-A-1,267,311 makes known a wine-making vat which is divided into two parts by means of a horizontal partition, the lower part forming the actual wine-making vat in which the alcoholic fermentation takes place, and the upper part forming a tun into which the juice is periodically transferred in order subsequently to descend and thereby sprinkle the marc head. The apparatus also includes a riser column, via which the juice passes from the vat towards the tun, a washer having a hydraulic lock, via which the juice can flow from the tun towards the vat, at the same time sprinkling the marc head, and finally a hydraulic valve by means of which the carbon dioxide gas contained in the vat can escape to atmosphere.
Since the hydraulic lock of the washer and that of the valve have previously been filled with water or juice, the carbon dioxide gas obtained as result of the alcoholic fermentation causes an increase in the pressure of the gas atmosphere prevailing in the vat above the grapes. The juice rises via the riser column and overflows into the upper tun. When the pressure of the carbon dioxide gas exceeds the value corresponding to the maximum height of the hydraulic lock of the valve, the carbon dioxide gas escapes directly to atmosphere, thus resulting in a pressure drop in the vat, and the juice contained in the tun flows via the washer towards the fermentation vat, at the same time sprinkling the marc head.
A disadvantage of this apparatus is that the juice is permanently covered by the carbon dioxide gas. In fact, the juice present in the tun is covered solely by the carbon dioxide obtained as result of the degassing taking place in the tun, this covering of carbon dioxide gas itself being in contact with the atmospheric air. Thus, during the movements of the juice in the tun and especially during discharge via the washer, there occurs uncontrolled oxygenation of the juice which can be detrimental to the subsequent quality of the wine. In fact, even if juice oxygenation phases are carried out in some processes for making red wine, these phases must be perfectly controlled in terms of the flow rate and aseptic quality, in order to govern the secondary fermentations and prevent extraneous seeding with undesirable aerobic bacteria, such as, for example, acetic bacteria. Furthermore, this arrangement leads, in the tun, to a loss by evaporation of ethyl alcohol and of the aromatic elements fundamental for the organoleptic characteristics of the wine.
FR-A-2,489,709 and FR-A-2,489,837 make known wine-making vats which, like that in FR-A-1,267,311, comprise a fermentation vat and an upper tun for the transfer of juice and for sprinkling or washing the marc head. The tun is emptied either by means of a flexible diaphragm valve, as in FR-A-2,489,709, or by means of a remote-controlled valve, as in FR-A-2,489,837.
In these documents, the rise of the juice is obtained by means of a column connecting the vat to the tun, the latter having previously been placed under a vacuum by means of a pump and a set of valves arranged on top of the tun. When the level in the tun reaches a set value, a detection system puts the tun in communication with the atmospheric pressure, thereby stopping the rise of the juice which can redescend into the vat, at the same time sprinkling the marc head, either by the automatic opening of the diaphragm valve or by the controlled opening of the communication valve.
The design of the tun and its functioning are such that, when the vacuum is cancelled, the juice, while it is falling through the washing port, sucks up air via the grating putting the tun in communication with the atmosphere, thus presenting the same risks of uncontrolled oxygenation and seeding as in FR-A-1,267, 311. Furthermore, these apparauses make use of complicated equipment, such as, for example, a vacuum pump, a set of shutters and valves, etc.